E-mail spam refers to unsolicited email messages that are sent by “spammers” to large numbers of recipients, few of whom want to receive them. Spamming is undesirable in many ways, including that it costs recipients time to delete the messages, and requires email service providers to provide resources to distribute and/or store the generally unwanted messages.
To detect currently known senders of spam, email service providers maintain lists of Internet Protocol (IP) addresses, referred to as blacklists. Messages from blacklisted IP addresses are (to a high probability) from spammers, and may be handled differently from other messages, e.g., the messages may be not accepted for distribution.
Spammers get around such blacklists by regularly changing their IP addresses. Each time a spammer changes an IP address, the email server accepts the email message, and uses CPU time to determine whether the mail being sent is spam, in which event it is automatically filtered into a junk mail folder or the like. Only after repeated processing of spam messages from such an IP address is that IP address added to the blacklist of “Bad” IP addresses on the receiving server. When this happens, the spammer switches to yet another IP address, and the process repeats.